A conventional tuner will be described below.
In the conventional tuner as shown in FIG. 4, input terminal 1 receiving a high-frequency digital signal is connected to high-frequency amplifier 2 for L band, the output of this high-frequency amplifier 2 is fed into bandpass filter 3 allowing L band frequency to pass through, and the output of this bandpass filter 3 and the output of local oscillator 4 oscillating a fixed frequency are fed into mixer 5.
The output of mixer 5 is connected to bandpass filter 6 allowing V band frequency to pass through. The output of bandpass filter 6 is connected to one terminal 7a of electronic changeover switch 7, while the output of tuning filter 10, to be described later, is connected to the other terminal 7b. 
Input terminal 1 is connected also to high-frequency amplifier 8 for V band, the output of this frequency amplifier 8 is fed into bandpass filter 9 allowing V band frequency to pass through, and the output of this bandpass filter 9 is fed into tuning filter 10.
Common terminal 7c of changeover switch 7 is connected to one input of mixer 12, while the other input thereof is coupled with the output of local oscillator 11.
The output of mixer 12 is fed into 39-MHz bandpass filter 13 and the output of this bandpass filter 13 is connected to output terminal 14.
Further, local oscillator 11 is loop-connected to PLL circuit 15 and this PLL circuit 15 is supplied with data from data terminal 16. The output of PLL circuit 15 is also applied to the tuning terminal of tuning filter 10 via local oscillator 11.
Operation of the broadband tuner configured as above will be described below.
When changeover switch 7 is turned to side 7a in order that L band is received, a high-frequency signal of L band supplied from input terminal 1 is passed through bandpass filter 3 and mixed with the output of local oscillator 4 in mixer 5 to be converted into V band frequency. This signal is passed through bandpass filter 6 and then mixed with the output of local oscillator 11 in mixer 12, whereby the signal is channel-selected and, at the same time, converted into intermediate frequency of 39 MHz. The intermediate frequency is put out from output terminal 14 via bandpass filter 13 whose center frequency is 39 MHz. Here, the oscillation frequency of local oscillator 11 is controlled in PLL circuit 15 based on data supplied to data terminal 16; namely, the channel selection is performed based on data supplied to data terminal 16.
On the other hand, when changeover switch 7 is turned to side 7b in order that V band is received, a high-frequency signal of V band supplied from input terminal 1 is passed through bandpass filter 9 and a desired wave is selected in tuning filter 10 for eliminating interfering waves. The center frequency of tuning filter 10 is varied by the output of PLL circuit 15 so that a desired frequency and frequencies in its vicinity are passed therethrough.
The desired frequency is mixed with the output of local oscillator 11 in mixer 12, whereby the frequency is channel-selected and, at the same time, converted into intermediate frequency of 39 MHz. This intermediate frequency is passed through bandpass filter 13 whose center frequency is 39 MHz and put out from output terminal 14. Here, the oscillation frequency of local oscillator 11 is controlled by PLL circuit 15 based on data supplied to data terminal 16; namely, the channel selection is performed based on data supplied to data terminal 16.
Incidentally, the L band frequency here denotes radio waves having a bandwidth of approximately 300 MHz centering about 1.4 GHz and the V band frequency here denotes radio waves having a bandwidth of approximately 100 MHz centering about 200 MHz.
In the conventional configuration as described above, however, since it operates as a single tuner in signal reception of V band, it required to have tuning filter 10 to eliminate image frequency interference. When such tuning filter 10 is employed, enormous amounts of manpower were required for adjustments of the same. More specifically, since the output of PLL circuit 15 is applied to local oscillator 11 and tuning filter 10 having their respective variable-capacitance diodes different from each other, frequency variations in separate diodes had to be adjusted with the same voltage from PLL circuit 15 by using inductance or the like, which took much time and hence there was the problem of increased manpower.